Metalloproteins containing manganese in a redox active role are involved in a variety of physiologically important reactions involving dioxygen metabolism. These include, amongst others, a superoxide dismutase that detoxifies superoxide radicals to O2 and peroxide, a catalase that disproportionates peroxide to O2 and H2O, and perhaps the most complex and important, the Mn-containing oxygen-evolving complex (Mn-OEC) that is involved in the oxidation of water to dioxygen in photosystem II. Oxygen, that supports all aerobic life, is abundant in the atmosphere because of its constant regeneration by photosynthetic water oxidation by the Mn-OEC. The light-induced oxidation of water to dioxygen is one of the most important chemical processes occurring on such a large scale in the biosphere. Photosynthetic water oxidation involves removal of four (4) electrons, in a stepwise manner by light induced oxidation, from two water molecules by the Mn-OEC to produce a molecule of oxygen. Central questions that need to be resolved and the overall objective of this proposal are as follows: 1) Determine the oxidation states and electronic structure of the Mn complex in the four intermediate S-states, 2) Characterize the structural changes of the oxo-bridged tetranuclear Mn complex as it advances through the enzymatic cycle, 3) Determine the mechanism of water oxidation and oxygen evolution, 4) Elucidate the structural and functional role of the cofactors Cl- and Ca2+. The interplay between X-ray spectroscopy (XAS) and EPR has played an essential role in our understanding of the structural and mechanistic aspects of O2 evolution. The samples for X-ray spectroscopy will be characterized by EPR. The structural changes of the Mn complex as it advances through the enzymatic cycle are determined by high-resolution XAS methods using samples prepared by flash illumination, and single-crystals of PS II. The oxidation states and electronic structure of the Mn complex are determined by Mn K- and L-edge, Kb emission, and X-ray resonant Raman spectroscopies.